Problem using input in Dictionary.. Help

[ThomasL]

You are overwriting weight for each aa in the input and calculating the sum of an int
so either:

prot_weight = {'A':89, 'V':117, 'L':131, 'I':131, 'P':115, 'F':165, 'W':204,
               'M':149, 'G':75, 'S':105, 'C':121, 'T':119, 'Y':181, 'N':132, 
               'Q':146, 'D':133, 'E':147, 'K':146, 'R':174, 'H':155}
prot_sequence = input('please write the amino acid sequence = ')
weight = 0
for aa in prot_sequence:
    weight += prot_weight[aa]
print(weight)

or do it the pythonic way:

prot_weight = {'A':89, 'V':117, 'L':131, 'I':131, 'P':115, 'F':165, 'W':204,
               'M':149, 'G':75, 'S':105, 'C':121, 'T':119, 'Y':181, 'N':132, 
               'Q':146, 'D':133, 'E':147, 'K':146, 'R':174, 'H':155}

prot_sequence = input('please write the amino acid sequence = ')
weight = sum(prot_weight[character] for character in prot_sequence)
print(weight)